Former Prime Minister Margaret Thatcher was a keen napper
Naps were much favoured by Winston Churchill, Margaret Thatcher and John F Kennedy.
But while a short snooze may revitalise tired brains, there is mounting evidence that dozing too long is harmful to health.
The Tokyo team pooled data from dozens of previous studies involving nearly 300,000 people.
They analysed health status according to how long volunteers napped for.
The results, presented at a diabetes conference in Munich, showed those kipping an hour or more in the day were 45 per cent more likely to get type two diabetes than those not napping.
Shorter snoozes of up to 40 minutes had no effect.
A 2014 Cambridge University study found hour-long naps increased the chances of premature death by almost a third.
The biggest risks appeared to be from lung diseases, such as bronchitis, emphysema and pneumonia.
Scientists said it’s possible napping triggers harmful inflammation in the body.
Health chiefs warned earlier this week Britain is in the grip of a diabetes epidemic
But other studies have suggested a quick doze may slash the risk of heart attacks and strokes by more than a third.
Scientists behind the latest findings said it’s possible some people nap in the day as their night-time sleep is disrupted by sleep apnoea – a snoring condition that is also a risk factor for diabetes and heart disease.
Almost one in ten adults is now affected – a record total of 3.8m in England alone
In a report on their findings they said: “Short naps less than 30 minutes increase alertness and should finish before the onset of deep slow-wave sleep.
“Entering deep slow-wave sleep and then failing to complete the normal sleep cycle can result in a phenomenon known as sleep inertia, in which a person feels groggy, disoriented, and even sleepier than before napping.”
Prof Naveed Sattar from Glasgow Uni said: “Napping may be an early warning sign of diabetes.
“We need trials to determine if how long we sleep makes a difference to real health outcomes.”
